Method and apparatus for non-contact, in-situ temperature measurement of a substrate film during chemical vapor deposition of the substrate film

ABSTRACT

A method and apparatus for the non-contact in-situ temperature measurement of a material layer during chemical vapor deposition of the material on an underlying substrate are provided. Magnitude modulated UV light having a plurality of separated spectral components is directed at the material being deposited on the substrate. The modulated UV light has a plurality of wavelengths corresponding to different temperature dependencies of absorptance in the deposited material. The separated spectral components are within transparency spectral windows of a plasma media contained in the CVD reactor. A portion of the magnitude modulated UV light is directed as a reference into a comparison device, such as a spectrophotometer. Light reflected from the deposited material is also directed at the comparison device for comparison with the reference light. That is, the magnitudes of the magnitude modulated components of the reflected light and the reference light are compared at more than one spectral component. The temperature of the deposited material is derived from this comparison. The results of the comparison are then utilized to control the temperature of the substrate on which the material is being deposited.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to the formation of a materiallayer by chemical vapor deposition (CVD) and, in particular, to a methodand apparatus for the non-contact, in-situ measurement of thetemperature of a diamond film, or other film material, during itschemical vapor deposition on an underlying substrate.

[0003] 2. Description of the Related Art

[0004] It has long been known that diamond can be used in many diverseapplications. In particular, diamond is finding increased use in theelectronics industry.

[0005] Given the relative scarcity of natural diamond, there has beenfor some time a concerted effort to synthesize diamond in thelaboratory. More recently, it has been found that it is possible tocommercially produce polycrystalline diamond film using chemical vapordeposition (CVD) techniques.

[0006] Chemical vapor deposition, as its name implies, involves agas-phase chemical reaction occurring above the surface of a solidsubstrate, which caused deposition of a CVD material film onto thesubstrate surface.

[0007] All CVD techniques for producing diamond films require some meansof activating gas-phase carbon-containing precursor molecules. Thisgenerally involves thermal or plasma activation, or the use of acombustion flame. While each technique may differ in its details, theyall share common features. For example, CVD growth of diamond normallyrequires that the substrate on which the diamond film is grown bemaintained at a predefined temperature during the CVD process.

[0008] One of the problems associated with manufacture of CVD diamond isthat the higher growth rates required for commercial feasibility areachieved only with a corresponding decrease in diamond film quality. Toa great extent, this loss of quality results from a lack of precisetemperature control of the deposited material during the CVD process.

[0009] Therefore, it would be highly desirable to have available atechnique for measuring and, thus, for controlling the temperature ofdeposited diamond film during the CVD process.

SUMMARY OF THE INVENTION

[0010] The present invention provides a method and apparatus for thenon-contact, in-situ temperature measurement of a layer of materialduring chemical vapor deposition (CVD) of the layer of material. Inaccordance with the invention, magnitude modulated ultraviolet (UV)light having a plurality of separated spectral components is directed atthe deposited material. The modulated UV light has a plurality ofwavelengths corresponding to different temperature dependencies ofabsorptance in the deposited material. The separated spectral componentsare within transparency spectral windows of a plasma media contained inthe CVD reactor. A portion of the magnitude modulated U light isdirected as a reference into a comparison device, such as aspectrophotometer. The light reflected by the deposited material is alsodirected from the CVD reactor into the comparison device. That is, themagnitudes of the magnitude modulated component of reflected light andthe reference light are compared at more than one spectral component.From this comparison is derived the temperature of the depositedmaterial. The results of the comparison are then utilized in a feedbackloop to control the temperature of the deposited material, typically bycontrolling the temperature of the substrate on which the material isbeing deposited.

[0011] Those skilled in the art will appreciate that the techniques ofthe present invention can be used for materials other than diamond byadjusting the spectral windows of light in the algorithm for derivingthe deposited film temperature based on results of the magnitudecomparisons for that particular material and CVD conditions.

[0012] A better understanding of the features and advantages of thepresent invention will be obtained by reference to the followingdetailed description and accompanying drawings that set forth anillustrative embodiment in which the principles of the invention areutilized.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a graph illustrating absorptance of light of differentwavelengths in a silicon substrate at three different silicon substratetemperatures.

[0014]FIG. 2 is a pictoral illustration of a method for the non-contactmeasurement of temperature in a CVD diamond process in accordance withthe concepts of the present invention.

[0015]FIG. 3 is a pictoral illustration of error sources that can beintroduced into the FIG. 2 method.

[0016]FIG. 4 is a block diagram illustrating a system for thenon-contact measurement of temperature in a CVD diamond process inaccordance with the concepts of the present invention.

DETAILED DESCRIPTION

[0017]FIG. 1 shows that the absorptance α of light of a given wavelengthλ in a silicon substrate will vary depending upon the temperature (T) ofthe silicon substrate. This fact is equally applicable to othermaterials, such as deposited diamond. Thus, a similar graph can beconstructed for deposited diamond, and for a great many other materialstypically formed by chemical vapor deposition (CVD). In accordance withthe present invention, this fact is utilized to provide a method andapparatus for the non-contact, in-situ measurement of the temperature ofa CVD diamond or other film.

[0018] From FIG. 1, it can be seen that absorptance a is a function ofboth wavelength λ and temperature T. That is, for a given wavelengthλ_(n)

α(λ_(n) ,T)=_(α)λ_(n)(T)=α_(n)(T)

[0019]FIG. 2 illustrates the general concepts of the present invention.As shown in FIG. 2, magnitude modulated UV light L from a light source100 is directed at a CVD deposited material film 102 forming on anunderlying substrate 104. The magnitude modulated UV light includes aplurality of separated spectral components with wavelengthscorresponding to different temperature dependencies of absorptance ofthe deposited film 102. As discussed in greater detail below, theseparated spectral components are selected to be within the transparencyspectral windows of the plasma media contained in the CVD reactor inwhich the film 102 is being deposited.

[0020] Further in accordance with the present invention, but not shownin FIG. 2, a portion of the magnitude modulated UV light is directed toa comparison device 106, such as a spectrophotometer. Light reflectedfrom the film 102 which is being deposited in the point of interest inthe CVD reactor is also directed to the comparison device 106. Thecomparison device 106 compares the magnitude, i.e. L·(1−α(λ,T)), of themagnitude modulated component of the reflected light and the magnitudeL, of the reference light at more than one spectral component, asdiscussed above. Based on a comparison of the reflected light and thereference light, an algorithm is utilized to determine the temperatureof the deposited film 102. The results of the comparison are, thus,utilized in a feedback mode to control the temperature of the depositedfilm 102, typically by controlling the temperature of the substrate 104upon which the film 102 is being deposited.

[0021] In accordance with the present invention, the algorithm utilizedfor determining the temperature of the deposited film 102 compensate forpotential errors that can be introduced into the calculation of the filmtemperature. For example, as shown in FIG. 3, parameters that must beconsidered in determining the temperature of the film 102 include filmthickness h, light 108 caused by the heat of radiation of the film 102,light 109 that comes to the film 102 from other sources 110 as reflectedoff the film 102 to the comparison device 106, and light absorptance inthe media 111 contained in the CVD reactor. As stated above, theseerrors can be eliminated by choosing an appropriate source 100 of lightL in appropriate operating wavelengths.

[0022] The last source of error, the fact that the temperature measuredis the temperature of the deposited film 102, which is slightly higherthan the Temperature T of the underlying substrate 102, can be avoidedby appropriate calibration of the temperature control mechanism.

[0023] More specifically, with reference to FIG. 3, the total lightincoming to the comparison device 106 will include the followingcomponents:

[0024] (1) light reflected from the film 102;

[0025] (2) dispersed light reflected from the film 102, which willdepend on the film temperature T_(f) and the film thickness h;

[0026] (3) light reflected by the substrate 104 on the boundary betweenthe film 102 and the substrate 104 and dispersed by the film 102, whichwill depend on the substrate temperature T, and the thickness of thefilm h;

[0027] (4) light secondarily-reflected from the walls of the CVD reactorand then by the film 102 (the effect of this component may be small if,for example, the bright spot of incoming light is small and/or thegeometry of the CVD reactor is right; this component may also be takeninto account with a correction factor or calibration);

[0028] (5) light radiated by the hot environment (e.g., plasmatron's,arc, hot walls, etc.) and re-reflected by the film 102; and

[0029] (6) light that is partially lost in the plasma that fills the CVDreactor chamber.

[0030] Considering these components, light of a given wavelength λincoming to the comparison device 106 can be described as:

L _(λ) ^(in) =KL _(λ) ^(out)[1−α_(λ)(T _(f))−β_(λ)(T _(S))α_(λ)(T_(f))]γ_(λ) +W _(λ)

[0031] where:

[0032] K=geometry factor of reactor

[0033] T_(f)=film temperature

[0034] T_(S)=film/substrate boundary temperature

[0035] (1−γ_(λ)=absorptance in CVD plasma

[0036] W_(λ)=radiation from hot environment re-reflected by film 102.

[0037] To exclude W_(λ)from L_(λ) ^(in), a magnitude modulated lightsource (periodic, with frequency Ω) is used. By using for the analysisonly light modulated on the same frequency part of the output of thecomparison device 106, the radiation of the hot environment having noperiodic component of the same frequency Ω, W_(λ)is completely removedfrom the data analysis.

[0038] In other words, the first Fourier component of the output of thecomparison device 106 is utilized (or, simply, a high-pass filter isused to reject the DC component of the output). In this way, for theFourier component of the modulated light (or, the AC part of the outputof the linear comparison device 106):$L_{\frac{in}{\lambda}} = {{{KL}_{\frac{out}{\lambda}}\left\lbrack {1 - {\alpha_{\lambda}\left( T_{f} \right)} - {{\beta_{\lambda}\left( T_{s} \right)}{\alpha_{\lambda}\left( T_{f} \right)}}} \right\rbrack}\gamma_{\lambda}}$or${L_{\frac{in}{\lambda}} \div L_{\frac{out}{\lambda}}} = {{K\left\lbrack {1 - {\alpha_{\lambda}\left( T_{f} \right)} - {{\beta_{\lambda}\left( T_{s} \right)}{\alpha_{\lambda}\left( T_{f} \right)}}} \right\rbrack}\gamma_{\lambda}}$

[0039] The ratio

[0040]$R_{\lambda} = {L_{\frac{in}{\lambda}} \div L_{\frac{out}{\lambda}}}$

[0041] is the result measured by comparison device 106.

[0042] As stated above, the plasma absorptance (1−γ_(λ)) introducesadditional uncertainty into the results of the analysis. Plasmaabsorptance γ_(λ)depends on light wavelength λ. Since the plasmatypically has transparency windows in the visual as well as the UVspectral areas, where γ_(λ)=1. Thus, choosing the wavelength λ of theincoming light to be within transparency windows eliminates this errorcomponent; thus

Rλ=K[1−α_(λ)(T _(f))−β_(λ)(T _(S))α_(λ)(T _(f])

[0043]FIG. 4 shows a system for measurement of the substratetemperature. As shown in FIG. 4, light L from a light source 100 can bedirected to the deposited film 102 via an optical system that includes aquartz window 112 in the chamber wall 114 of the CVD reactor 116. FIG. 4also shows a portion of the light L from the light source 100 beingdirected to the spectrophotometer 106. The light reflected from thedeposited film 102 is directed back through the quartz window 112 andthe optical system and to the spectrophotometer 106 for comparison withthe reference light L. The temperature determined by thespectrophotometer system 106 is then utilized to control the temperatureof the substrate 104 on which the film 102 is deposited, therebycontrolling the temperature of the deposited film 102. Those skilled inthe art will appreciate that the temperature control mechanism can beany well-known and conventional system suitable for this purpose andconnected between the spectrophotometer 106 and the substrate 104.

[0044] With respect to the algorithm utilized by the spectrophotometersystem 106 to determine film temperature, reference is make to theequations provided above. The term R_(λ)is a measured value; theunknowns are K,α (T_(f)), β_(λ)(T_(S)). Actually, in a linearapproximation

Rλ=K[1−α^(o) _(λ)(T _(f))−hα_(λ)(T _(f))−hβ(T _(S))α_(λ)(T _(f))]

[0045] where:

[0046] K=geometrical factor

[0047] h=film thickness

[0048] α_(λ)(T)=film absorptance

[0049] β(T)=film/substrate boundary reflectance

[0050] and α^(o) _(λ)(T), α_(λ)(T), β(T) are known calibrated functionsof T.

[0051] Given that α^(o) _(λ)(T), α_(λ)(T) and β(T) are known calibratedfunctions of T, we have the following four unknown variables in theequilibrium for R_(λ)K, h, T_(S) and T. Therefore, four equilibria areneeded to solve for these four unknowns. Thus, for measurements R_(λ1),R_(λ2) R_(λ3) and R_(λ4) 2t four different wavelengths are needed.

[0052] In some instances, the geometric factor K may be pre-calibratedfor a particular CVD reaction after installation of the temperaturecontrol system. If K is know, then only three variables need be defined,requiring only three measurements at three different wavelengths.

[0053] To solve the system at equilibria, one can use, for example,computer or look-up tables.

[0054] As indicated in FIG. 4, all light transmissions can be carried byoptical fibers, i.e. between the light source and the CVD reactor,reference light between the light source and the spectrophotometer and,in the case of the reflected light, from the CVD reactor to thespectrophotometer.

[0055] It should be understood that various alternatives to theembodiment of the invention described herein may be employed inpracticing the invention. Thus, it is intended that the following claimsdefine the scope of the invention and that methods and apparatus withinthe scope of these claims and their equivalents be covered thereby.

What is claimed is:
 1. A method of non-contact, in-situ temperaturemeasurement of a substrate during chemical vapor deposition of amaterial on the substrate in a reactor vessel, the method comprising:directing at the deposited material magnitude modulated UV light havinga plurality of separated spectral components with wavelengthscorresponding to different temperature dependencies of absorptance inthe deposited material, the separated spectral components being withintransparency spectral windows of a plasma media contained in the reactorvessel; directing a portion of the magnitude modulated UV light to acomparison device as reference light; directing light reflected from thedeposited material to the comparison device; utilizing the comparisondevice to compare the magnitude modulated components of the reflectedlight and the magnitude modulated components of the reference light todetermine a temperature of the deposited material; and utilizing thedetermined temperature to control the temperature of the substrate. 2.An apparatus for the non-contact, in-situ temperature measurement of asubstrate during chemical vapor deposition (CVD) of a material film onthe substrate in a CVD reactor vessel that contains a plasma media, theapparatus comprising: a light comparison device; a light source thatprovides magnitude modulated UV light having a plurality of separatedspectral components with wavelengths corresponding to differenttemperature dependencies of absorptance in the material film, theseparated spectral components being within transparency spectral windowsoff the plasma media contained in the reactor vessel; an optical systemthat directs a first portion of the magnitude modulated light at thematerial film, directs a second portion of the magnitude modulated lightat the light comparison device, and directs light reflected from thematerial film at the light comparison device, and wherein the lightcomparison device compares the second portion of the magnitude modulatedlight and the light reflected from the material film to determine atemperature at the material film; and a temperature control system thatutilizes the determined temperature to control the temperature of thesubstrate.